The present invention relates to an arrangement for the control of the power transmission of a four-wheel drive motor vehicle with a main drive axle provided with a cross differential and with an auxiliary drive axle driven by way of a longitudinal clutch continuously controllable by means of an adjusting member, and in which a control unit receives input signals from transmitters or pick-ups detecting the velocity of the wheels at the auxiliary drive axle.
According to the German patent application P 36 08 059.4, corresponding to U.S. Pat. application 07/024,068 filed Mar. 10, 1987, an arrangement for the control of the power transmission of a four-wheel drive vehicle is provided which achieves with a relatively small number of sensor-detected operating and driving parameters and with low technical control expenditures that the traction-conditioned advantages of the four-wheel drive can be combined with the advantages of the driving behavior of a two-wheel drive without having to accept the respective disadvantages thereof. Such a vehicle is described in the article by H. Bott and M. Bantle, "Der Porsche Typ 959 - Gruppe B - ein besonderes Automobil" ["The Porsche Type 959 - Group B - A Special Automobile"], Part 2, ATZ 88 (1986), Volume 6, pages 353 to 356.
This is achieved primarily in that a traction force proportion Fzf of the wheels at the auxiliary drive axle is determined by way of an amount of a desired traction force Fzs multiplied by a distribution factor; the same is determined from a desired power output Ps (selected by the driver) and by a vehicle velocity vf. The distribution factor is thereby determined from a traction force excess which is calculated from the difference of the desired traction force Fzs and the driving resistance Fv of the vehicle.
It is the object of the present invention to provide an arrangement for the control of the power transmission to the wheels of a four-wheel drive vehicle which refines the driving-dynamic overall-behavior of a motor vehicle attainable with the arrangement according to the P 36 08 059.4-12 and which in particular further improves the load change behavior during curve drives and the longitudinal stability of the vehicle.
The underlying problems are solved according to the present invention in that the control unit produces a control magnitude for the adjusting member of the longitudinal clutch in such a manner that it is in a functional relationship with at least one activating magnitude which is determined from the vehicle velocity determined from the velocity of the wheels at the auxiliary drive axle, whereby within a first low velocity range (0 smaller than or equal to vf smaller than or equal to vfb), the activation is cancelled and within a second higher velocity range (vfa smaller than vf smaller than or equal to vfb), the activating magnitude is raised from beginning with an increasing velocity to a control magnitude value and remains at this control magnitude value above the second velocity range (vf greater than vfb).
The advantages of the present invention reside in the first instance in that the driving-dynamic overall-behavior of a motor vehicle is further improved because the arrangement reacts better to internal (for example, load-change) and external (for example, abruptly changing friction value conditions in the wheel-/road surface-contact) interference magnitudes, stabilizes the vehicle especially when driving through curves and altogether acts in a quieting manner on the overall driving behavior and improves the longitudinal and cross-stability.
This was achieved in the first instance in that a continuously controllable longitudinal clutch of a four-wheel drive motor vehicle is activated with a characteristic dependent on velocity. The activation is thereby suppressed in a low velocity range, is raised in a higher velocity range with increasing velocity starting from 0 to a control magnitude value and is kept constant above this higher velocity range. Velocity ranges and control magnitude values are additionally variable in the control unit by input of an adjusting magnitude (driving program).